Process for making paper

ABSTRACT

Paper is made by mixing anionic starch, carboxyl methyl cellulose or other polymeric binder into a cellulosic thin stock together with a cationic inorganic or polymeric coagulant and then flocculating the suspension by means of an anionic swelling clay or other anionic retention aid.

This invention relates to processes for making paper (by which weinclude paper board), and in particular processes of making paper whichis strengthened by starch.

It is standard practice to make paper by a process comprisingflocculating a cellulosic suspension by the addition of a high molecularweight, polymeric, retention aid, draining the flocculated suspensionthrough a wire to form a wet sheet, and drying the sheet.

One particular class of paper-making processes are microparticulateprocesses in which the flocculation with polymeric retention aid isfollowed by degrading the flocs by agitation and then reflocculating bythe addition of a microparticulate material, such as bentonite.

It is well known to include low molecular weight cationic polymer in thesuspension, either by addition at the thick stock stage or subsequently,in various paper-making processes for various purposes. It is also wellknown to include inorganic coagulants such as poly aluminium chloride oralum for various purposes. Reference is made to, for instance, U.S. Pat.No. 4,913,775 for a description of various processes and, in particular,a microparticulate process sold under the trade name Hydrocol.

It is known to add cationic starch to the cellulosic suspension inpapermaking processes as a strengthening aid, and in some processes italso contributes to retention. Processes have also been described whichcomprise addition of raw, untreated starch to the cellulosic suspension.Processes in which starch is added to the cellulosic suspensiongenerally tend to have the disadvantage that particular care must betaken to ensure good retention of starch so that there are notsignificant levels of dissolved or undissolved starch in the whitewaterdraining through the wire. See for instance WO95/33096.

Processes are described in GB 2,292,394 in which anionic starch, carboxymethyl cellulose or other polymeric binder capable of hydrogen bondingto cellulose are added to the thin stock with a cationic polymer whichhas a molecular weight above 150,000, preferably 1 million or more andwhich insolubilises the anionic binder. Cationic starch can also beadded.

Processes are described in WO93/01353 in which an anionic retention aidbased on starch, a cellulosic derivative or guar gum free of cationicgroups and an aluminium compound are added to the suspension. Anotherdisclosure of processes in which an anionic compound and a low molecularweight cationic polymer are added to the suspension is in JP-A-03193996.

Although various processes which are known can be optimised to giveuseful strength in the dry sheet and can be optimised to givesatisfactory short drainage times and/or good retention of the fibresand/or the binder, it would be desirable to be able to provide a processwhich gives optimum utilisation of the binder in the sheet (and thusoptimum strength) together with good retention of the binder, the fibresand the fines in the cellulosic suspension, and good drainageproperties.

It might have been thought that these objectives could be achieved bymodifying the process described in GB 2,292,394 by adding a highmolecular weight cationic polymeric retention aid to the suspension, butwe have found that this does not give any significant or usefulimprovement.

According to the invention, a process for making paper (including paperboard) comprises providing a thin stock suspension of cellulosic fibres,mixing into this suspension (a) a water soluble anionic or non-ionicpolymeric binder and (b) a water soluble cationic material selected fromwater soluble organic polymeric coagulants having intrinsic viscositynot more than 3 dl/g and inorganic coagulants,

then flocculating the suspension by mixing into the suspension ananionic retention aid (which may be a microparticulate anionic retentionaid),

draining the flocculated suspension to form a wet sheet, and

drying the wet sheet.

We have surprisingly found that the addition of the anionic retentionaid, instead of traditional cationic polymeric flocculants, afteraddition of the binder and cationic polymeric coagulant, gives goodflocculation of the suspension and subsequently a marked improvement inthe drainage rate and good retention of fibre and fines. Further, itdoes not lead to any significant deterioration in the retention ofbinder and so gives good retention of the binder.

The cellulosic suspension may be any conventional thin stock formed fromany conventional cellulosic feed, including recycled feed material. Thethin stock may be substantially unfilled (i.e., without the deliberateaddition of significant amounts of filler) or it may be filled.

The binder is a water soluble material capable of substantial hydrogenbonding with cellulose. That is, it is capable of binding with thecellulose fibres in the paper stock, for instance to levels of at least1 or 2% (dry binder based on dry stock), often with a binder retentionof at least about 60 or 70 or even 80%. In practice the binder needs tobe non-ionic or anionic, since if it is cationic then the binding of thebinder to the cellulosic fibres will predominantly be due to thecationic groups rather than due to hydrogen bonding. In order thathydrogen bonding predominates, the non-ionic or anionic binder willnormally be a polyhydroxy material. In order that it acts as a binder inthe final sheet, thereby increasing the strength of the sheet, it mustbe polymeric and of high molecular weight. Thus the molecular weightwill normally be in excess of 5,000, and often in excess of 50,000 andgenerally above 100,000.

In practice, the polymeric binder is usually a cellulosic compound, anatural gum or a starch, but it can be a synthetic polymer such aspolyvinyl alcohol. Natural and modified natural polymers includecellulosics, gums and starches, for instance carboxymethyl cellulose,xanthan gum, guar gum, mannogalactans and, preferably, anionic starch.The binder preferably has a pendant ionisable group which is generallysulphate, carboxylate or phosphate. Suitable starches include oxidisedstarch, phosphate starch and carboxy methylated starch.

The amount of binder is normally at least about 1% (dry weight binderbased on dry weight suspension) and can be up to, for instance, 10%.Generally it is 1 to 8%, preferably around 3%, for instance 3 to 5%(i.e., 30 to 50 kg/t).

The cationic material is preferably a cationic polymeric coagulant whichhas IV not more than 3 dl/g. In this specification IV is intrinsicviscosity measured by suspended level viscometer at 25° C. in 1N sodiumchloride buffered to pH 7. Preferably IV is not more than 2 dl/g, forinstance 1.5 dl/g or below. Normally it is at least 0.1 or 0.5 dl/g.Preferred cationic polymeric coagulants have high charge density, forinstance above 3 meq/g and usually above 4 meq/g.

Inorganic coagulant such as aluminium compounds, for instance polyaluminium chloride, can be used alone as the water soluble cationicmaterial, or in combination with the polymeric coagulant.

The preferred cationic polymeric coagulants are materials such aspolyethylene imines or polyamines (both preferably being fullyquaternised), dicyandiamide condensation polymers (usually beingsubstantially fully quaternised or in salt form) and polymers of watersoluble ethylenically unsaturated monomer or monomer blend which isformed of 50 to 100 mole percent cationic monomer and 0 to 50 molepercent other monomer. The amount of cationic monomer is usually atleast 80 to 90 mole percent, and homopolymers are often preferred.Ethylenically unsaturated cationic monomers that can be used includedialkylaminoalkyl (meth)-acrylates and -acrylamides (usually inquaternary or other salt form) and diallyl dialkyl ammonium chloride,for instance diallyl dimethyl ammonium chloride (DADMAC). Particularlypreferred polymers are DADMAC homopolymers and copolymers.

When the polymer is a copolymer, the comonomer is usually acrylamide, orother water soluble non-ionic ethylenically unsaturated monomer.

The cationic polymeric coagulant may be a linear polymer. Alternativelyit may be produced in the presence of multifunctional additives whichproduce structure, for instance polyethylenically unsaturated monomerssuch as tetraallyl ammonium chloride, methylene bis acrylamide andmultifunctional monomer included in the polymer chain. The amount ofthese additives, if used, is generally at least 10 ppm and usually atleast 50 ppm. It may be up to 200 or 500 ppm.

The amount of cationic material is normally an excess over that amountwhich is required to give observable retention when the anionicretention aid is added. The amount may be sufficient to cause thesuspension to have a zeta potential which is around zero or is positive,but satisfactory retention is often obtainable even though the zetapotential is slightly negative. In practice, the amount of cationicmaterial is best determined by forming a thin stock containing thedesired amount of the binder (having regard to the strength propertiesthat are required) and then observing the retention effect upon addingthe retention aid after adding various amounts of the cationic material.

It is usually undesirable for the cationic material to include anysignificant amount, or indeed any amount, of high molecular weightcationic polymeric material (for instance intrinsic viscosity above 4dl/g) since the use of such a material does not usually cause anyimprovement in performance, provided sufficient cationic material whichis inorganic and/or low molecular weight has been used. However, ifdesired, other materials can be added with or after the cationicpolymeric or inorganic coagulants discussed above, provided these extramaterials do not interfere with the process.

The amount of cationic polymeric coagulant is normally from 0.25 to 10kg active polymer per ton dry cellulosic suspension, preferably 1 to 3kg/t.

In the process the binder may be added prior to the cationic coagulantor after the cationic coagulant. The binder and coagulant may be addedessentially simultaneously. The coagulant may be added as a single doseor as a split dose, for instance partially before and partially afterthe binder. The order of addition of the binder and cationic coagulantcan be varied as convenient without significant deterioration inresults.

After treatment of the suspension with the binder and cationic polymericcoagulant, the anionic retention aid is mixed into the treatedsuspension. This mixing may be done under medium or high shear, but isnormally done under sufficient force simply to mix the anionic retentionaid into the suspension, for instance at the headbox or prior to it.

The amount of anionic retention aid is normally 0.5 to 10 kg/t drycellulosic suspension, preferably 1 to 4 kg/t.

The anionic retention aid is a material which acts to flocculate thetreated thinstock suspension and thus improve the drainage in comparisonwith a non-flocculated treated thinstock suspension.

It may be a substantially water soluble anionic polymeric material andthus it may be, for instance, a material as described in WO98/29604.

Preferably, however, it is a microparticulate anionic retention aidwhich may be inorganic or organic. For instance, it may be an organicanionic microparticulate retention aid such as described in U.S. Pat.Nos. 5,167,766 and 5,274,055. Preferably it is an inorganic anionicmicroparticulate retention aid. Such materials are well known andinclude swelling clays, generally referred to as bentonite, colloidalsilica, polysilicic acid, polysilicic acid or polysilicate microgels,and aluminium modified versions of these. Mixtures may be used, e.g., oforganic and inorganic microparticles.

Preferably no additional components are added to the cellulosicsuspension after treatment with binder and cationic coagulant and beforeaddition of anionic retention aid.

After treatment with the anionic retention aid the flocculatedsuspension is drained through a wire to form a wet sheet. The wet sheetis then dried in standard manner to form a dry paper (including paperboard) sheet.

In the process the retention of binder in the sheet is preferably atleast 60 or 70%, more preferably at least 80%, and even 85 or 90% orabove.

In the invention we also provide the use of an anionic retention aid asdiscussed above to improve the drainage of a cellulosic suspension whichhas been treated with binder and cationic polymeric coagulant, of thetypes discussed above.

In the process we often find that drainage times for a given volume ofbackwater can be reduced to 70 or 60% of drainage times under equivalentconditions but without addition of anionic retention aid, and may evenbe reduced to below 50 or 40% of these times.

The invention will now be illustrated with reference to the followingexamples.

EXAMPLES

For each test 1 liter of cellulosic stock was used, at a concentrationof 0.5% solids. For each process anionic starch was added as the binderat a level of 3% followed by Polymer A at the dosage given in the tablesbelow. In some tests subsequent materials were added in the dosagesgiven below in the tables.

Polymer A was a polyDADMAC homopolymer of IV about 1 dl/g.

A drainage test was carried out and the time for collection of 600 ml ofbackwater was measured. This was the drainage time. Results are shown inTables 1 to 3 below.

TABLE 1 Evaluation of single addition of Polymer A Starch Dosage PolymerA Dosage Drainage time (%) (kg/t) Active (seconds) 3 0 20 3 0.8 7 3 1.67 3 3.2 11

TABLE 2 Effect of a high molecular weight flocculant Polymer A DrainageStarch dosage (kg/t) Flocculant addition time Dosage (%) active (g/t)active (seconds) 3 1.6 0 7 3 1.6 low cationic-200 8 3 1.6 mediumcationic-200 9 3 1.6 low anionic-200 6

TABLE 3 Effect of sodium bentonite Polymer A Drainage Starch activeBentonite dosage time Dosage (%) (kg/t) (kg/t) active (seconds) 3 1.6 07 3 1.6 1 2 3 1.6 2 3

It can be seen that good drainage results are obtained with the use ofPolymer A alone, and no significant improvement is seen with thesubsequent addition of various high molecular weight flocculants.However, when sodium bentonite is added after the Polymer A, there is asignificant improvement in the free drainage time, to values much lowerthan expected.

EXAMPLE 2

These tests show the good starch retention which is obtained using thesystem of the invention. In this test the same furnish as in Example 1is used. To this is added anionic starch at a level of 3% dry starch ondry fibre. Subsequently a cationic coagulant is added. In some systems(those of the invention) a further component, the anionic retention aid,is then added. Dosages and results are shown in Table 4 below.

TABLE 4 Anionic Coagulant Retention Retention Starch Dosage Aid (if AidDosage Retention Coagulant (kg/t) used) (kg/t) (%) Polymer B 0.6 67 0.890 1.2 93 1.6 94 2.0 86 2.4 85 3.6 84 Polymer B 1.2 Sodium 2.4 91 0.6bentonite 2.4 81 1.2 1.2 91

Polymer B is a polyDADMAC homopolymer of IV about 2 dl/g.

What is claimed is:
 1. A process for making paper comprising providing athin stock suspension of cellulosic fibres, mixing into the suspension(a) a water soluble anionic or non-ionic polymeric binder and (b) awater soluble cationic material selected from water soluble organicpolymeric coagulants having intrinsic viscosity below 3 dl/g and acharge density of above 3 meq/g, and inorganic coagulants, thenflocculating the suspension by mixing into the suspension an anionicretention aid, draining the flocculated suspension to form a wet sheet,and drying the wet sheet.
 2. A process according to claim 1 in which theanionic or non-ionic polymeric binder is a binder capable of substantialhydrogen bonding with cellulose.
 3. A process according to claim 1 inwhich the polymeric binder is selected from cellulosic compounds,natural gums, starches and polyvinyl alcohol.
 4. A process according toclaim 1 in which the polymeric binder is selected from anionic starchand carboxyl methyl cellulose.
 5. A process according to claim 1 inwhich the cationic polymer is selected from polyethylene imines,polyamines, dicyandiamide polymers, and polymers of water solubleethylenically unsaturated monomer or monomer blend comprising 50 to 100mole percent cationic monomer and 0 to 50 mole percent other monomer. 6.A process according to claim 5 in which the cationic polymer is apolymer of monomers comprising water soluble polyethylenicallyunsaturated monomer.
 7. A process according to claim 1 in which thewater soluble cationic material is a polymer of 80 to 100% diallyldimethyl ammonium chloride and 0 to 20% acrylamide and having intrinsicviscosity below 3 dl/g.
 8. A process according to claim 1 in which theanionic retention aid is selected from inorganic swelling clays,colloidal silica, polysilicic acid and silicate microgels, aluminiummodified colloidal silica or polysilicic acid or polysilicate microgels,and organic microparticles.
 9. A process according to claim 1 in whichthe binder is selected from anionic starch and carboxy methyl cellulose,the cationic material is a polymer of diallyl dimethyl ammonium chloridehaving intrinsic viscosity below 3 dl/g and the anionic retention aid isan inorganic swelling clay.